The invention relates to a drive arrangement, especially for a hoist mechanism.
The drive arrangement for the hoist mechanism or the propulsion mechanism of a crane or the like usually comprises a three-phase motor which is united with a friction brake to form a constructional unit. The friction brake is controlled so that it engages under spring loading and brakes the motor shaft, when the three-phase motor is switched off, while the three-phase motor is switched on the friction brake is cleared and releases its shaft. Thus unintended falling of the load of a hoist mechanism or unintended driving of the propulsion mechanism, for example on current failure, are prevented.
In one known drive arrangement the three-phase motor is formed as sliding armature motor, the armature of which is mounted axially displaceably, together with the motor shaft, in the motor housing. The motor shaft carries a brake cone and is stressed axially by a spring into a brake drum, likewise of cone form, fast with the housing. The armature of the motor is likewise of cone form and is pressed by the spring out of the stator opening, which likewise is of cone form. The stator field generated when the motor is switched on draws the armature into the stator opening, whereby the brake cone is released at the same time. The known drive arrangement has the advantage that no additional controlling is necessary for the operation of the friction brake. Admittedly this advantage is gained at the cost of a comparative high construction expense of the sliding armature motor. Furthermore, the conical form of the stator opening necessitates a comparatively large stator diameter, if saturation phenomena of the stator iron are to be avoided. Compared with three-phase current motors with cylindrical armature, sliding armature motors therefore have comparatively large dimensions, which is undesired for a plurality of practical causes.
It is further known to use as hoist mechanism drive a three-phase motor with axially fixed cylindrical rotor and to provide the friction brake, united with the three-phase motor into one unit, with a separate electromagnet which is energized together with the motor and thus releases the brake which is engaged when the motor is switched on. The electromagnet of the brake is energized through a rectifier circuit, which is accommodated in a motor terminal box united with the motor and containing the terminals of the phase windings of the three-phase motor.
In order to render possible two different working speeds of the hoist mechanism, the three-phase motor has a two-speed pole-changing three-phase field winding which is connected through connection leads with a separate, remotely placed control box or switch box. The switch box contains, for the change of direction of rotation of the three-phase motor, at least one rotation direction control contractor which exchanges the phases by pairs in each case, and for the variation of speed of rotation at least one speed control contactor alternately switching on the two field windings, which are dimensioned for different pole numbers. In the known drive arrangement in addition to the rotation direction control contactor and the speed control contactor a brake control contactor is provided in the remote switch box, which through a plurality of connecting leads for the one part connects the alternating current side of the rectifier circuit arranged adjacent to the three-phase motor, with the three-phase current mains and on the other hand is connected with one of its switch contacts on the direct current side in series with the electromagnet of the friction brake. For the controlling of the brake the known drive arrangement thus requires several additional connecting leads between the switch box and the rectifier circuit arranged in the motor terminal box. Since especially in hoist mechanism applications the hoist mechanism is mobile, for example on a crane trolley, over relatively great distances of several tens of meters in relation to the switch box, the connecting leads additionally necessary for the controlling of the brake, between switch box and motor terminal box considerably increase the production expense.